[Race] Fixing AFHTTPRequestOperation.HTTPError being overreleased.

Just discovered that the following loop _always_ results in a crash in my app

``` objc
for (int i = 0; i < 10000; i++) {
    // fire up a JSON request with an unknown URL resulting in a 404 status code
}
```

due to `AFHTTPRequestOperation.HTTPError` being overreleased by arc. An easy fix for that is to make AFHTTPRequestOperation.HTTPError __atomic__ (which makes sense anyway because it is being used on different threads).

* I think I remember another issue encountering this crash which I haven't been able to find as of this writing. I'm wondering if we could get a fast merge and version bump because Crashlytics told me that this crash happened quiet often in real world code.
* Maybe as a second step, we should consider making all properties which are being used by AF on different threads  atomic.